(1) Field of the invention
The present invention relates to lightweight structural materials and more particularly to lightweight structural materials produced from coal combustion by-products and materials and processes for making them.
(2) Description of the Related Art
The burning of coal for the generation of steam and electricity creates a large amount of residue in the form of ash. Each year in the United States over one hundred million tons of coal combustion ash is generated and must be used or disposed of in some manner. Most of the ash is now disposed of by on-site landfilling However, some small part of the ash is used as a component in building materials. Potential uses for coal combustion ash have been surveyed by the American Coal Ash Association and include structural fills, embankments, mine reclamation, underground mine backfilling, and cement--concrete, among others.
The physical and chemical composition and characteristics of coal combustion ash can vary depending not only upon the type of coal that is burned, but also upon the type of combustion equipment and the conditions of combustion. Several of the common types of coal ash are class F fly-ash (ASTM C-618-89, Class F; finely divided residue produced from burning anthracite or bituminous coal); fluidized bed combustion (FBC) ash (produced from burning coal with limestone in a fluidized bed combustion furnace); and class C fly ash ASTM Designation No. C618-89, a finely divided residue produced from burning lignite or sub-bituminous coal and having a lime content that is typically higher than 10%). More complete descriptions of the types, properties and composition of various types of coal combustion ash have been provided in, for example, Material and Research Society Proceedings, Volumes 43, 113, 136 and 178; Management of High Sulfur Coal Combustion Residues: Issues and Practices, pp. 285, Chugh, Y. P. and G. A. Beasley, Eds., Conference Proceedings, Dept. of Mining and Mineral Resources Engineering, Southern Illinois Univ., (1994); Fly Ash and Coal Conversion By-Products: Characterization, Utilization and Disposal I, McCarthy, G. J., and R. J. Lauf, Eds., Material Research Society Symposia Proceedings, vol. 43 (1984); Fly-ash and Coal Conversion By-Products: Characterization, Utilization and Disposal IV, McCarthy, G. J. and F. P. Glasser, Eds., Material Research Society Symposium Proceedings, Vol. 113 (1987); Fly-ash and Coal Conversion By-Products: Characterization, Utilization and Disposal V, Hemmings, R. T., E. Berry and G. J. McCarthy, Eds., Material Research Society Symposium Proceedings, Vol. 136 (1988); and Fly-ash and Coal Conversion By-Products: Characterization, Utilization and Disposal VI, Day, R. L. and F. P. Glasser, Eds., Material Research Society Symposium Proceedings, Vol. 178 (1989).
One well known use for coal combustion ash is as a component of a cementitious material for use in cement-concrete as a building material, a rigid fill material, for road construction, or other similar applications. Class C fly ash has been used far more in these applications than Class F, or other coal combustion ash, due to its higher lime content (usually measured as calcium oxide) and, thus, its property of being a cementitious, as well as a pozzolanic (cement-like) material. For example, U.S. Pat. No. 2,250,107, to Nelles, disclosed that fly ash could be used to replace all or part of sand as the aggregate in a cement-based concrete. Jones et al., in U.S. Pat. No. 2,382,154, teach that the addition of a minor amount of an anhydrous alumino-silicate material, such as fly ash, to a major amount of an alumino-silicic acid material, such as provided by shales, slates and clays, and further mixed with an alkaline earth base and water provided a synthetic stone having improved compressive and flexural strength.
More recently, Dunstan, Jr., in U.S. Pat. No. 4,256,504, taught a composition that included a major portion of a fly ash having a calcium content greater than about 20% and a minor portion (i.e., about 5% to 15%) of calcium sulfate. Cook et al., in U.S. Pat. No. 4,680,059, disclosed the production of a building material based on a reactive amorphous silicate that also included an aqueous solution of an iron salt, lime, reinforcing fillers that could be fibers, and from 10% to 50% of a filler containing reactive polyvalent cations that could be fly ash, among other materials. Also, Brook et al. in U.S. Pat. No. 5,536,310, disclosed a cementitious composition that was composed of a cementitious material such as a hydraulic cement, fly ash--preferably Class C fly ash having a high calcium oxide content--and a hydroxycarboxylic acid or salt thereof. However, compositions using Class F fly ash rather than Class C fly ash were also disclosed. The composition was reported to have enhanced strength and durability and reduced permeability while not sacrificing early strength or retarding the set time.
Styron, in U.S. Pat. No. 5,714,002, disclosed a process for making blended hydraulic cement compositions containing sub-bituminous (Class C) fly ash having a lime content of at least about 21% and preferably at least 25%, a retarding agent, citric acid and an alkali source such as potassium carbonate. Other compositions were disclosed where the components just described were blended with a second fly ash that was selected from lignite fly ash, bituminous fly ash and scrubber material. Another composition having a blend of Class C fly ash with a filler material, that could be Class F fly ash, was disclosed by Bennett et al. in U.S. Pat. No. 5,106,422. The composition was composed of water mixed with a major amount of the filler material and a minor amount of the Class C fly ash. The material was reported to have rapid setting properties and to have controllable permanent strength properties.
In U.S. Pat. No. 5,374,307, Riddle discloses a composition for forming construction blocks and encapsulating hazardous materials that includes a pozzolanic fly ash, such as Class C fly ash, mixed with bottom ash and water. The composition can be used as a mortar, or could be compressed and formed into blocks.
Yet more reports of methods and compositions that utilized fly ash as one component in some type of hardened building material are provided in U.S. Pat. Nos. 5,482,549; 5,520,730; 5,565,028; 5,573,588; 5,578,122; 5,580,378; 5,601,643; and 5,622,556.
The production of structurally useful materials containing coal combustion ash other than Class C or Class F fly ash has also been reported. For example, the use of fluidized bed combustion ash (FBC ash) in a process for producing hardened materials was described by Mitsuda et al. in U.S. Pat. No. 5,100,473. Fluidized combustion ash was mixed with water and the mixture was kneaded to form granules. The granules could then either be mixed with an additional amount of powdery combustion ash, or could be treated by steam at atmospheric pressure to obtain hardened material. This material could be broken to obtain a crushed stone-like material for use as a roadbed material.
A potentially attractive use for structural materials that are based upon coal combustion ash is in the production of support posts, beams, crib members, and the like that could be used to replace wooden support members in, for example, underground mines. Wood has proven to be a desirable material for this application due to its high strength-to-weight ratio and its failure mode that is characterized by a crackling sound and continuing to bear a significant load even after initial failure. A significant amount of wood of sound structural quality is now used as mine support members. Its availability, however, is becoming more and more scarce, and the availability is often seasonal. If some suitable replacement for wood were available, the wood now used for mine supports could go to some other, potentially higher value, uses. Furthermore, since the wood supports now placed in mines are generally left in place after mining activities have ceased, they are lost to further use and wasted. Additional problems caused by wooden support members is that wood for this purpose is seasonally available and a scarcity is risked unless an inventory is maintained. Another drawback is that wooden support members are combustible and, thus, increase the fire hazard in the mine.
The use of materials containing high levels of coal combustion ash in mine support members would be particularly advantageous in that it would not only free a significant amount of wood for other, more valuable uses, but would permit the return of a significant amount of coal combustion ash to its source in a useful, safe and non-polluting form. In fact, several reports disclose work in this area. For example, a yieldable confined core mine roof support has been reported by Frederick in U.S. Pat. No. 5,308,196. The support included a yieldable container such as a cylindrical corrugated metal pipe that is filled with a compressible filler such as volcanic pumice, fly ash, cinders and light and heavy aggregate, among others. Morgan, in U.S. Pat. No. 5,573,348, described structural members formed from a cement-based slurry infiltrated fiber material. The cement-based slurry could include a cement-fly ash blend and the structural members were formed by adding the slurry to a pre-formed bed of fibers that substantially filled a mold so that the slurry completely infiltrated the spaces between the fibers.
Numerous methods and compositions are known that incorporate various types or blends of coal combustion ash into structural or building materials. But each type of ash presents some unique property that must be dealt with in order to obtain materials that have useful strength and durability. For example, the relatively high sulfate content of FBC ash causes undesirable swelling during curing and also reduces the durability of cementitious materials in which it is a component. Thus, its level in the material must be limited to provide a swelling strength and durability that is acceptable in the particular application of interest.
In Class F fly ash, the presence of a significant amount of unburned carbon causes an increased water requirement for mixing the mortar. This increased water, in turn, significantly reduces the compressive strength of the cured mortar.
Accordingly, it would be desirable to provide a method for using significant amounts of coal combustion byproducts, particularly high volume byproducts such as fly ash and fluidized bed combustion ash, for the production of structural materials. It would also be desirable to be able to produce these structural materials from coal combustion ash while avoiding at least some of the disadvantages normally associated with low calcium content and high unburned carbon and/or high levels of sulfates in the ash. It would be particularly desirable if these structural materials were suitable for the production of support members that could be used in coal mines to replace wooden support members and that would have strength-to-weight properties and a failure mode similar to that of wood. Furthermore, it would be desirable if these structural members produced from coal combustion ash were durable, non-polluting, fireproof and capable of being cut with a saw and of accepting screws and lag bolts and the like.